The Nigerian government has accused opposition politicians and election monitors of trying to foment a military coup by demanding a rerun of the weekend's tarnished presidential election.

As a trickle of early results puts the ruling People's Democratic party in the lead, opposition parties called for the election to be annulled because of ballot rigging and chaotic organisation. They said they would call street demonstrations and launch a legal challenge if, as expected, the PDP candidate, Umaru Yar'Adua, was declared the winner today.

Nigeria's vice president and opposition candidate, Atiku Abubakar, described the election as a "national tragedy". "My impression is that this is a sham. It is no election ... They have no alternative than to cancel the election altogether."

Nigeria's main election monitoring group also called for a rerun after large numbers of people were unable to vote because of a shortage of ballot papers in some opposition strongholds. "These were not credible elections," said Innocent Chukwuma, chairman of the Transition Monitoring Group, which had 50,000 observers. "We are going to call for a rerun of elections. You cannot use the result from half of the country to announce a new president."

The head of the electoral commission, Maurice Iwu, said he did not see serious problems with the vote. "The poll we had was free and fair. Nobody was molested," he said. But European Union monitors say they are worried about the conduct of the election after witnessing ballot box stuffing, violent intimidation and shortages of voting papers. At least 16 people were killed in election day violence.

Turnout also appeared low for a presidential election, which the opposition interpreted as evidence of a government strategy to prevent opposition supporters from voting. The head of the EU monitors, Max van den Berg, said: "For now the assessment is outspokenly negative ... I'm very concerned."

The president of the Nigerian senate, Ken Nnamani, said the flawed election would spawn bitterness and joined the call for a fresh vote. "There will be a legacy of hatred. People will hate the new administration and they will have a crisis of legitimacy," he told Reuters. "These people have no shame. We are not encouraging other African countries who look up to us for an example. We have abdicated that role."

It is the first time since Nigeria's independence in 1960 that one civilian president has handed over power to another. For much of its history, Nigeria has endured rule by military leaders who plundered much of its oil wealth in cooperation with many civilian politicians.

The government accused Mr Nnamani, whose position puts him third in line to the presidency, of stirring up chaos so that the military can seize control and install him in power. Nigeria's information minister, Frank Nweke, issued a statement describing critics of the election as "plotters" who wanted to "get the international community to label Nigeria as a failed state and also incite the rank and file of the Nigerian armed forces with the sole aim of scuttling the presidential elections".

The government also blamed "coup plotters" for a failed attempt to attack the electoral commission headquarters in the capital, Abuja, using a petrol tanker as a bomb.

Mr Nnamani dismissed the accusations. "In my capacity as head of the National Assembly, I have no reason to do anything unconstitutional. These are trumped-up charges, trying to involve me in a civilian coup," he said. "What I am saying is let us give the people the right to vote for a new government. I am not part of anything and I don't think there is any such plot."

To avoid a runoff, the candidate with the most votes must win at least 25% of the votes in 24 of Nigeria's 36 states.